Qorvo SiC FET body diode characteristics and good SW characteristics

Qorvo's SiC FET product line is built on a core technology of high-voltage normally-on SiC JFETs and low-voltage normally-off Si MOSFETs in a cascode configuration.

Let's take a look at the differences between common SiC MOSFETs and Qorvo's body diodes. Figure 1 shows a simple cross-sectional structure of a general SiC MOSFET and Qorvo's cascode-connected SiC FET.

Figure 1: Schematic cross-sectional diagram of a typical SiCMOSFET and Qorvo SiC FET

First, the body diode of a typical SiC MOSFET is made of SiC and is a PN diode. Since SiC, a wide bandgap material, is used, the VF is high, exceeding 3.5V. Also, the Qrr characteristics are not very good, and the value increases very quickly depending on the temperature. When used in a step-down power supply, synchronous rectification or a Schottky barrier diode in parallel is required. Even with synchronous rectification, as the switching frequency increases, losses during the dead time period required to avoid shoot-through current become significant, eventually requiring a parallel Schottky barrier diode. Additionally, the body diode of a typical SiC MOSFET may suffer from internal crystal defects that cause the VF to rise during use.

However, Qorvo 's SiC FET has a body diode made of Si. The current passes through the body diode of the low-voltage MOSFET, and then through the two resistors RJFET and RDRIFT of the JFET. Therefore, the SiC​ ​JFET itself behaves like a simple resistor. Cascode-connected Si MOSFETs have a low breakdown voltage of 20 to 30V, so the body diode characteristics are very good. As shown in the graph in Figure 2, the forward voltage drop is typically around 1.5V due to the low voltage drop.

Figure 2: Qorvo SiC FET third quadrant characteristics

In the waveform shown in Figure 3, the Qrr of the SiC​ ​MOSFET was measured at 150°C, and a relatively high value of 220nC was observed. Also, the body diode of the SiC MOSFET has a high forward drop voltage VF, so if VF is too high, connect a SiC Schottky barrier diode in parallel, but the Qrr will rise to 275nC.

Qorvo's SiC FET has a Qrr of only 105nC at 150℃, so it can contribute to reducing switching loss.

The waveform data in Figure 3 was measured at 150°C, 800V, and 11A. Qrr of the body diode of Qorvo 's SiC FET is characterized by not changing much even if the temperature changes, and Qrr only increases by 10% when changing from 25℃ to 150℃. The reason for this is that most of the Qrr of Qorvo 's SiC FET is generated from the output capacitance (Coss) of the SiC JFET. Since it is a capacitive charge, it does not increase with temperature. Some Qrr comes from the body diode of the Si MOSFET and increases with temperature, but the overall increase is only 10%.

Figure 3: Body diode recovery characteristics

Although it is a summary, Qorvo company's SiC FET When conducts from source to drain, SiC JFET has very low on-resistance and low voltage Si MOSFET The body diode voltage drop of 1.5V Because it is low, SiC MOSFET or GaN HEMT The voltage drop will be much better and smaller than the voltage drop of .

Therefore, even when used as a step-down power supply, the synchronous rectifier circuit FET If you are not driving a parallel Schottky barrier diode, you can simply use the internal body diode. Moreover, this body diode SiC not, Si So there is no need to worry about deterioration.

figure Four is shown in Si fast recovery diode (FRD), Qorvo company SiC FET body diode, SiC MOSFET The forward effect voltage of the body diode of VF This is a comparison. twenty five ℃ and 150 The case of ℃ is shown. Qorvo company SiC FET The body diode of Si fast recovery diode (FRD) You can see that it is equivalent to

Figure 4: Comparison of body diode forward voltage drop between SiC MOSFET and cascode structure SiC FET

For SiC MOSFETs and GaN MEMTs, reverse recovery losses can be sufficiently low or even non-existent. However, the voltage drop during reverse conduction is a different story; the voltage drop during reverse conduction causes large losses during switching dead time.

Qorvo 's SiC FET has very good body diode characteristics, and has the advantage of lower switching loss compared to SiC MOSFETs and Si MOSFETs.

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